It is well known to provide adaptive echo cancellation arrangements in systems in which signals are transmitted in both directions via a two-wire line to which a transmitter and a receiver are both coupled via a hybrid circuit. It is desirable for the echo canceller in such arrangements to cancel substantially completely components of signals transmitted by the transmitter which would otherwise be coupled to the receiver due to imperfect balancing in the hybrid circuit.
Conventional echo cancellation arrangements do not compensate for non-linear distortions of the signals to be cancelled. Non-linear distortions arise, for example, from conversions between analog and digital signals as a result of quantization and clipping.
In the above respect, Tol et al. U.S. Pat. No. 4,746,902, issued May 24, 1988 and entitled "Arrangement For Compensating For Non-Linear Distortion In An Input Signal To Be Digitized And An Echo Cancelling System Comprising Such An Arrangement", describes an echo cancellation arrangement in which the digital output of an analog-to-digital converter, which is supplied with a received signal via a hybrid circuit, is summed with compensation values stored in a memory which is addressed by the output of a quantizing circuit. The quantizing circuit is driven by a linear estimate of the echo signal or by the received signal in analog or digital form. Such an arrangement does not address non-linear distortions due to clipping, and requires increasing memory sizes for increasingly accurate compensation and/or smaller quantization steps.
Transmission systems using multicarrier modulation are becoming increasingly important. The principles of multicarrier modulation are described for example in "Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come" by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5-14, May 1990. The term "multicarrier modulation" is used to include modulation in various different ways, for example using transforms such as an FFT (Fast Fourier Transform), DFT (Discrete Fourier Transform), DCT (Discrete Cosine Transform), or a wavelet transform. The term "discrete multitone" (DMT) is used to refer to multicarrier modulation which, as is desirable, is effected using a Discrete Fourier Transform.
An example of a transmission system using multicarrier modulation is an ADSL (asymmetric digital subscriber line) system. For example, an article by S. Fleming et al. entitled "ADSL: The on-ramp to the information highway", Telephony, Jul. 12, 1993, pages 20-26 describes an ADSL system in which four asymmetric 1.5 Mb/s channels are provided for transmission in a downstream direction from a telephone CO (central office) to a subscriber, in addition to various data channels and POTS (plain ordinary telephone service) carried symmetrically (i.e. bidirectionally), via a two-wire telephone subscriber line.
The patent application by John M. Cioffi et al. referred to above also describes an ADSL transmission system using multicarrier modulation.
It has also been proposed to provide an echo cancellation arrangement in an ADSL system or in other transmission systems using multicarrier modulation. In this respect, the patent application in the names of M. Ho and J. M. Cioffi referred to above describes a particular form of echo cancellation arrangement, as does an article by M. Ho et al. entitled "High-Speed Full-Duplex Echo Cancellation For Discrete Multitone Modulation", 1993 International Conference on Communications, pages 772-776, May 1993.
Transmission systems using multicarrier modulation typically have a transmitted signal with a higher peak-to-average ratio than single carrier transmission systems, and thus generally require digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) with higher precision, i.e. more bits. With any given DAC and ADC, the signal level must be adjusted to make use of the full conversion range. An excessive signal level results in clipping noise that is generally impulsive. Conversely, signal levels that avoid all clipping result in a waste of the ADC/DAC precision most of the time, the quantization process then giving rise to a significant amount of quantization noise. A tradeoff must therefore be made between the amount of clipping and the amount of quantization noise.
The above comments also apply, of course, to other transmission systems in which clipping and quantization occur for example due to the use of DACs and ADCs.
As indicated above, the clipping noise and quantization noise are not cancellable using conventional echo cancellation arrangements, and thus detract from the performance of the transmission system. Clipping noise and quantization noise also adversely affect the performance of the far end receiver, whether or not echo cancellation is present. An object of this invention, therefore, is to provide a method of mitigating clipping and/or quantization effects in a digital transmission system, and to provide an improved terminal for such a transmission system.